1. Field of the Invention
The invention relates to a new blocked diisocyanate, a method of manufacturing the diisocyanate, and use of the diisocyanate for powder coating compositions.
2. Discussion of the Background
All aliphatic diisocyanates blocked with epsilon-caprolactam are either highly viscous or have a low melting point. Therefore they are not candidates for the manufacture of polyurethane (hereafter referred to as "PUR") powders.
The only economically important blocked diisocyanates for producing weather-resistant PUR powder coatings are epsilon-caprolactam adducts of isophoronediisocyanate (IPDI). Epsilon-caprolactam blocked IPDI melts at 53.degree.-55.degree. C. This low melting temperature results in caking of the powder when stored. To increase the melting point, IPDI is subjected to chain lengthening with a polyol (NCO:OH=2:1) prior to the blocking with epsilon-caprolactam. In Ger. No. OS 21 05 777, polyols such as 1,1,1,-tris(hydroxymethyl) propane; 2,2,4-trimethylhexanediol; and diethylene glycol are mentioned as chain lengthening agents for IPDI. In Ger. No. OS 25 42 191, mixtures of di- and trifunctional polyols are mentioned as lengthening agents and in Ger. No. OS 31 43 060 aliphatic diamines are employed. Ger. Nos. OS 27 35 497 and OS 28 42 641 describe reactive PUR powders characterized by containing an epsilon-caprolactam blocked isocyanate-group-containing IPDI adduct as a hardener. Because IPDI in the abovementioned PUR powders is subjected to molecular augmentation prior to the blocking, resulting in "elimination" of NCO groups, the PUR powder hardener produced therefrom has a relatively low NCO content of 9 to at most 16 wt. %.
There continues to be a need for a completely or only partially caprolactam blocked aliphatic or cycloaliphatic diisocyanate with a high NCO content, which displays a high melting point, satisfactory storage stability, and good use properties, without molecular augmentation, while completely avoiding the disadvantages of customary blocked non-chain-lengthened diisocyanates as hardeners in powder coatings.